Crystal Density Calculator
The density a structure implies: ρ = ZM / NAV, from the six cell constants, the formula and the number of formula units in the cell. This is the figure a CIF records as _exptl_crystal_density_diffrn.
- You supply
- Six cell constants, a chemical formula — brackets and charges included, as on the CHN page — and Z, the number of formula units in the whole cell.
- Reading it
- This is a property of the model, not a measurement: it assumes the cell holds exactly Z formula units of what you typed, so disagreement with a measured density usually means Z is wrong or there is solvent in the crystal that is not in the formula. The volume per non-hydrogen atom lands near 18 Å3 for organic and organometallic structures only — metals and dense ionic solids sit well below it.
Worked examples: NaCl – halite (rock salt) · Cu – copper, face-centred cubic · α-Fe – ferrite, body-centred cubic · CsCl – caesium chloride · quartz – α-quartz, SiO2 · rutile – rutile, TiO2 · aragonite – aragonite, CaCO3 · ZrO2 – baddeleyite, monoclinic zirconia · albite – low albite, NaAlSi3O8
Input
Results
| Formula read as | NaCl |
|---|---|
| Formula weight | 58.4427 g/mol |
| Cell volume | 179.406 Å3 |
| Calculated density | 2.164 g/cm3 |
| Volume per non-H atom | 22.4 Å3 (8 non-hydrogen atoms in the cell) |
This is the calculated density — what a CIF records as _exptl_crystal_density_diffrn. It is a statement about the model, not a measurement: it assumes the cell holds exactly Z formula units of exactly what you typed. That is what makes it worth computing. Where it disagrees with a measured density the assumption is wrong somewhere — most often Z, or solvent that is in the crystal and not in the formula.
For an organic or organometallic structure the cell volume divided by the number of non-hydrogen atoms in it lands near 18 Å3, so a figure far outside that usually means Z is wrong rather than the cell. The rule is about molecular packing and does not carry over to metals or dense ionic solids — copper metal is 11.8 Å3 per atom and silicon 20.0, and both are perfectly ordinary.